For packages to be included in an End of Day form, they must have a ship date that's the same as the form's creation date. For instance, if you have a shipment with a ship date of 4/5/2018, it can only go on an End of Day form that also has a date of 4/5/2018. Additionally, the shipment must not have been on any previously created End of Day forms in ShipStation.
You can ensure that every shipment has the correct ship date by setting your Shipping Cutoff in the label Document Options.
Set Shipping Cutoff
Head to your Account Settings (the wrench icon in the upper right).
Next, select Printing and then Printing Setup from the sidebar on the left.
Click on Document Options for your Labels.
Scroll to the Shipping Cutoff section and set the time at which you'd like the ship date to move to the next day:
Additionally, when you actually ship the order, you can choose the ship date.
NOTE: You cannot change a ship date of a shipment after its label has been created. If your carrier supports voiding labels, you can void the incorrectly-dated shipment and then create a new label for that order. For this reason, we recommend that you configure the first two settings according to your schedule to avoid having to void labels.